Thermionic emitter and method of making same



E. A. COOMES ET AL THERMIONIC EMITTER AND METHOD OF MAKING SAME June 30, 1959 Filed Jan. 8, 1957 N wE CNN

INVENTORS EDWARD A.CO0MES AND FAY E. GIFFORD BY flm ATTOBEEYS @231 ZDDO United States Patent THERMIONIC EMITTER AND METHOD OF MAKING SAME Edward A. Cooines and Fay E. Gifford, Notre Dame, Ind., assignors to University of Notre Dame Du Lac, Notre Dame, Ind, a corporation of Indiana Application January 8, 1957, Serial No. 633,007

17 Claims. (Cl. 148-1.6)

This invention relates to an improved thermionic emitter and to a method for making this emitter, and, more particularly, to filaments of refractory metal and a methed for making these filaments.

In vacuum tubes, the source of free electrons is provided by a cathode element of a directly or indirectly heated type. In order to provide a suitable electron source in the relatively high temperature ranges encountered in tube operation, the cathodes are generally fabricated of highly refractory metals, such as molybdenum, tungsten, hafnium, or tantalum, in a filamentary form. These metals, in addition to providing acceptable electron emitting qualities because of their fairly low work function, are characterized by adequate mechanical strength in the temperature range between 1500 Kelvin and 3000 K. and by possessing low vapor pressures and high melting points. However, to improve the electron emitting characteristics of these filaments, the emitter metal is combined or coated with an electropositive impurity substance such as thorium, barium, caesium, and strontium, for instance, from the rare earth or alkaline earth metal groups.

In the case of thoriated tungsten, the tungsten metal is combined with thorium oxide or thoria during the metal working operations so that, when the metal is drawn into filaments, the greater portion of the thoria impurity is occluded as particles within the body of the filament. Following the formation of the tungsten wire into a filamerit, the filament is flashed by heating it in vacuum to a temperature around 2500 K. to convert a portion of the thoria into thorium which forms a monatomic layer on the outer surface of the filament. During normal use, the heating of the filament evaporates thorium from the surface ofthe filament which is replaced from thoria in the body of the filament, generally by movement of the thorium along grain-boundaries to the surface of the filament.

An additional effect produced by the addition of the thorium impurity to the tungsten metal, is that of retarding crystal development and of stimulating growth of a small grain structure to afford adequate passages for permitting movement of the thorium to the surface of the filament. .The tungsten metal, during working and drawing, is of a generally fibrous structure having some ductility to permit drawing, but this structure is converted to agranular structure of small grains by heating the filament above the equiaxing temperatureduring flashing in the presence of the thoria impurity. However, the small grain structure has the undesired result of making the tungsten filament, which is generally of the order of two to five mils in diameter, rather brittle and subject: to breakage in response to physical or thermally induced shocks. This fracture of the filament, which largely occurs along shear planes transverse to the lone gitudinal axis, of the lament, cc nts for a large pro, portion of vacuum tube failures.

Although the thoriation of tungsten improves its emitting characteristics, with a corresponding increase in the brittleness of the filament, the thoriated filaments are somewhat limited in the temperature ranges in which they can be used because tof the loss of surface thorium due to evaporation at a rate faster than it can be replaced by thorium from the thoria in the body of the filament. This evaporation loss is considerably reduced by carbonization of the thoriated filament which involves heating the thoriated filament in the presence of a hydrocarbon, such as naphthalene, benzene, or anthracene, to form a shell of tungsten carbide surrounding the filament. These filaments can be used in adverse environments in which positive ion bombardment or an oxidizing atmosphere is encountered, but these filaments are excessively brittle and extremely sensitive to shock.

Other proposals involving the use of electropositive materials in conjunction with refractory metals to provide electron emitting sources generally are limited in application by the temperatures involved or the need for avoiding envelope contamination. For instance, caesiated cathodes require the presence of a low pressure caesium vapor in the envelope which limits the use of this type of tube to relatively low voltage ranges in which envelope contamination can be tolerated. The caesiated cathode is also subject to deactivation by overheating which destroys the oxide layer on the cathode. In the case of oxide coated or Wehnelt cathodes, this type of emitter is not easily produced in requiring numerous treating steps in preparation and is somewhat sensitive to the presence of oxygen in the envelope.

Accordingly, one object of the present invention is to provide an improved method of producing thermionic emitters.

Another object is to provide an improved thermionic emitter.

Another object of this invention is to provide an improved filament of highly refractory metal.

A further object is to provide a method of forming new and improved thermionic emitting filaments of highly refractory metals.

Another object is to provide a new and improved filament of refractory metal having a selected torsional strain applied thereto, and a method of making such a filament.

Another object is to provide a filament of refractory metal comprising a single crystal, and a method of making such a filament.

A still further object of this invention is to provide a new and improved method'of growing av crystal in a filament of highly refractory metal.

Another object involves the provision of a refractory filament having an improved electron emission charac Another object is to provide a filament of refractory metal having improved resistance to shearing or fracture.

A still further object is to provide a method of forming single crystal filaments of refractory metals by subjecting the filament to torsional stress and heating.

In accordance with these and other objects, anembodiment of the present. invention comprises twisting a filament of a highly refractory metal, such as tungsten, tantalum, hafnium, or molybdenum, about its axis to provide a thermionic filament having an electron emission pattern varying in. accordance with the twist appliedjto the. filament. The twisted filament is then subjeete'd to heatunder selected conditions of vacuum and tempers ture and for selected periods of time to provide afilament of a monocrystalline nature possessing not only the improved electron emission pattern of the twisted filament but also an improved resistance to breaking. The increased physical strength of the recrystallized filaments is desirable in vacuum tubes wherein one major cause of tube failure arises from shearing or fracture of the filament due to thermally induced or physical shock. This type of tube filament destruction is particularly important because of the need for providing control and communication components of an electronic nature in modern jet aircraft wherein the tube filaments are subjected to shearing forces of a considerable magnitude during even normal flight conditions. A single crystal filament is also of value in afiording an improved electron source for making work function measurements.

Additional objects and advantages of the present invention will become apparent from the following detailed description thereof in the course of which reference is made to the accompanying drawing wherein:

Fig. l is a schematic diagram of one apparatus for making refractory metal electron emitters in accordance with the present invention; and

Fig. 2 is a schematic diagram of another type of apparatus used in making the improved emitter of the present invention.

In general, the present invention concerns new and improved thermionic emitters and methods for making these emitters. The emitter element comprises a length of filament of highly refractory metal, such as tungsten, molybdenum, tantalum, or hafnium, which is twisted about its longitudinal axis in suitable apparatus until a selected number of turns has been applied to the filament. The filament is then heated in vacuum at a temperature of from 1300 K. to 2900 K. The filament, at the completion of the heating operation, exhibits an electron emission pattern comprising a spiral of high emission having a number of turns equal to the number of turns mechanically applied to the length of filament. By suitably adjusting the temperattue and duration of the heating, a perfect crystal extending completely across the diameter of the filament and along the entire length can be produced. The heating of the filament can be accomplished either by heating the entire length of the filament or by using a gradient method of heating in which a temperature step is gradually moved along he length of the filament.

In forming the raw metal filaments, the refractory material is first fabricated, generally by using processes well known in powder metallurgy, to form stock which is then drawn to filamentary form of a suitable diameter, such as from two to five mils. The raw metal filament is then cut into suitable lengths for processing.

A length of filament is then subjected to torsional stress by twisting the filament about its own axis so that a predetermined number of turns per inch is provided therein. Although any suitable apparatus can be used, one convenient means of twisting the refractory filament comprises a device including means for fixedly securing one end of the filament and rotatable means to which the other end of the filament is secured. The rotatable means is then rotated until the filament has been given a desired number of turns per inch. With molybdenum and tungsten filaments of from two to five mils in diameter, the filaments have been found to shear upon the application of a stress in excess of thirty turns per inch. Ord narily filaments of this diameter are subjected to a torsional stress of between one to twenty turns per inch. Upon the completion of the twisting, the filament is re moved from the twisting device. The spring back of the filament accompanying its removal is generally less than three quarters of a turn and can be disregarded, although compensation for this spring back can be made during twisting by adding additional twist in the amount" of the. normally encountered spring back.

A twisted filament 8 of refractory material is then placed 111 the apparatus shown in either Fig. l or Big. I

2 for heating under vacuum conditions. The apparatus shown schematically in Fig. 1 comprises a projection tube apparatus of the type described in detail by C. S. Robinson in the Journal of Applied Physics, volume 13, pages 647 et seq. (1942). In general, this apparatus comprises a glass envelope 10 having a cylindrical coating or layer 12 of a material such as zinc silicate on the inner surface thereof. The twisted filament 8, at one end, engages a terminal 14 hermetically sealed in the envelope 10 and, at its other end, is secured to a light spring 16 of electrically conductive material which exerts a very small tensional force on the twisted filament 8 to hold the filament in electrical contact with the terminal 14. The spring 16 is connected to another terminal 18, which is hermetically sealed in the envelope 10.

To provide a means for heating the twisted filament 8 to a predetermined temperature, the terminals 14 and 18 are connected across a variable source of potential 26 in series with a suitable indicating instrument, such as an ammeter 22. The source 20 is then adjusted to heat the twisted filament 8 to a temperature between l300 K. and 2000 K. with molybdenum and between 2200 K. and 3000 K. with tungsten. To maintain the interior of the envelope It} at a substantial vacuum during heating, a vacuum pump 24 is placed in communication with the envelope 10 to maintain a vacuum of the order of 10* to 10- millimeters of mercury.

The emissivity of the twisted filament 8 is determined during heating by observing the emission pattern on the layer 12. In the case of twisted filaments having between one and twenty turns per inch, a spiral band of high electron emission having the same pitch as the twist applied to the filament 8 is observed on the layer 12. When the twisted filament 8 is removed from the envelope 10 following heating and is polished, either electrolytically or mechanically, and again heated, the screw pattern or axially extending spiral of high electron emission is again observed, thus establishing that the screw pattern is not a surface discontinuity and perhaps indicating that the pattern extends as a screw plane across the entire diameter of the filament. The screw pattern on the surface of the twisted filament 8 following heating is readily visible on a one hundred power optical microscope. The production of the spiral of high emission on the twisted refractory filament 8 is substantially independent of the duration of the heating and of the temperature, within the ranges specified above, and is directly dependent for its pitch on the turns per inch of torsional stress applied to the filament.

In order to recrystallize the twisted refractory filament 8 to a structure including one or more perfect crystals of substantial length, the heating of the filament 8 under the above conditions of temperature and vacuum is maintained for a selected period of time of the general order of twelve hours. The time of heating is not a critical parameter since crystal growth has been observed after heating for periods varying between ten and twenty hours. However, each specific type of filament develops optimum crystal growth after heating at a specific temperature with a selected amount of twist applied thereto. Generally, the longest crystals are produced in twisted molybdenum filaments with heating in the temperature range from 1600 K. to 1700 K. and are produced in twisted tungsten filaments with heating in the temperature range from 2400 K. to 2700 K. In the case of twisted molybdenum filaments of the type set forth above, what appears to be optimum crystal growth is produced with heating at 1650 K. and an applied torsional stress of four turns per inch.

Following the heating of the twisted filament 8 for an interval within the range set forth above, the growth of perfect crystals in the filaments of refractory metal can be visually observed 'on the projection pattern provided on the coating 12. The crystal growth is determined by a-s tudy of the thermionic emission pattern provided on the projection tube in which the length of the light and dark bands along crystal planes spaced by known angular separation is observed. This plot of emission against crystal direction is well accepted for establishing crystal identification in wires of body centered symmetry, and the refractory metals, molybdenum, tungsten, hafnium, and tantalum all are characterized by body centered symmetry. In the projection patterns obtained on the coating 12 from the twisted filament 8 set forth above, the dark band at the 110 crystal plane and the bright band of emission at the 111 and 116 crystal planes are readily apparent and indicate that single crystal lengths of up to fifteen centimeters are obtained. Although the degree of recrystallization obtained in different specimens varied over a range of single crystal lengths between two centimeters and fifteen centimeters, in every twisted filament 8, recrystallization to some extent is obtained, as contrasted with repeated failures in the prior art to obtain predictable crystallization in certain types of refractory metal filaments.

It should also be noted that the growth of single crystals in the twisted filaments 8 does not adversely affect the spiral of high emission noted above. In filaments having single crystal growth up to fifteen centimeters, the screw pattern of emission is observed. Laue X-ray patterns taken along the 110 plane of a single crystal grown in a filament of the type set forth above show a single crystal structure with imperfections which appears to suggest that the spiral emission pattern may be due to screw planes of impurity material passing through a single crystal. A possible explanation for this phenomenon is that grains of an electropositive impurity substance, such as thoria, are occluded in the stock during the swaging of the powdered base metal. These grains are then extended into long fibers during drawing so that, following twisting and heating, the impurity appears as a screw plane in the single crystal.

As set forth above, the twisted filament 8 can also be treated in the apparatus shown in Fig. 2 wherein it is subjected to a temperature gradient under selected conditions of vacuum. A method of treating filaments using a temperature gradient is described in detail by E. N. du Andrade in Proceedings of the Royal Society, volume 163A, pages 16 et. seq. (1937). The apparatus of Fig. 2 is somewhat similar to that shown in Fig. 1 and comprises a glass envelope 10a including a zinc silicate coating 12a and having a pair of terminals 14a and 18a between which the twisted filament 8, which is torsionally strained as described above, is connected by the use of a light, electrically conductive spring 1611. A vacuum pump 24a maintains the pressure within the envelope 10a in the order of from 10" millimeters of mercury to 10* millimeters of mercury. A variable voltage source 28a connected between the terminals 14a and 18a provides means for heating the filament 8 to a selected temperature, and a suitable instrument 22a, such as an ammeter, provides an indication of the energy supplied to the filament 8.

The source a is adjusted to provide heating of the twisted filament 8 to a temperature suitable for providing crystal growth and for producing the spiral path of high electron emission. For molybdenum wire of from two to five mils in diameter, a temperature range between 1300 K. and 2000" K. provides acceptable results, with a temperature of from 1600 K. to 1700 K. generally providing a consistent growth of single crystals. For tungsten filaments of from two to five mils in diameter, a temperature range of from2200 K. to 2900 has provided useful results, with a temperature in the range of from 2400 K. to 2700 K. generally providing a favorable recrystallization.

In the apparatus shown in Fig. 2, the entire length of the twisted filament 8 is not continuously subjected to heat, but rather a temperature step or gradient is gradually moved along the length of the filament. To accomplish this, the interior of the envelope 10a is filled with mercury 26 to a height suflicient to cover the entire length of the filament 8. A lower end of the envelope 10a is placed in communication with a mercury removal apparatus 28, and this apparatus is adjusted to remove the mercury 26 at a rate dependent on the length of time that the filament 8 is to be heated. Although different heating intervals produce crystal growth, a heating interval of twelve hours with the apparatus 28 adjusted to re move the mercury 26 at a rate sufiicient to lower the level between one and two centimeters per hour provides good recrystallization. The body of mercury 26 main--v tains the portion of the twisted filament 8 which is sub-. merged therein at substantially room temperature with the remaining portion of the filament 8 rising to the treating temperature. This provides a sharp temperature discontinuity at the upper surface of the mercury 26 which is gradually moved along the length of the twisted filament 8 as the level of the mercury is lowered.

As in the case of the twisted filament 8 treated in the apparatus shown in Fig. 1,- the twisted filament 8 treated in the apparatus of Fig. 2 under the conditions set forth above exhibits an axially extending spiral or screw pattern of high electron emission on the coating 12a having a pitch equal to the turns per inch of twist applied thereto. When these filaments are electrolytically polished or mechanically polished with a corresponding reduction in diameter of one or more mils, the spiral of emission remains apparent in the projection pattern.

The twisted filament 3 treated by the gradient method under the conditions set forth above exhibited pronounced crystal growth, which is again determined by observation of the bright and dark bands in the projection pattern along the 110, 111, 116 planes of the crystal. In five mil molybdenum filaments having only one turn per inch, perfect single crystals up to five centimeters in length were grown and, in similar filaments having ten turns per inch, perfect crystals extending the full lengths of the specimens were grown.

Broadly, the present invention provides an improvement filament of highly refractory metal having both resistance to shearing and fracture due to its monocrystalline structure and an evenly distributed emission pattern comprising an axially extending spiral of high electron emission. The emission characteristic and the recrystallization are provided in the method of the present invention by torsionally stressing the filament by twisting it about its longitudinal axis and then subjecting the filament to heating under varying conditions of time, temperature, and vacuum. The recrystallization and emission phenomenon occurs over varying ranges of time and temperature with the important control condition being. the application of the torsional stress. However, with particular respect to the recrystallization phenoiiienonpparticular conditions of temperature and torsional strain appear to provide optimum results for different types of wires or filament.

The invention is disclosed in detail by the following examples which are provided for the purpose of illustrating specific embodiments and methods but are not to be construed as limiting the scope of the invention.

EXAMPLE 1 Twisted molybdenum filament (Robinson apparatus) A ten inch length of unpolished five mil molybdenum filament obtained from Fansteel Metallurgical Corporation was subjected to a torsional stress of ten turns per inch and then introduced into the envelope 10 (Fig. 1). The envelope was evacuated to a pressure of 10*" millimeters of mercury and the filament was heated for a period of fifteen hours at a temperature of 1750 K. A study of the emission pattern at the end of this interval showed an axially extending spiral pattern of high emission having ten convolutions per inch. A study of the bright and dark bands of the projection pattern along 7 the 110, 111, and 116 crystal planes indicated a single crystal growth of three centimeters.

7 EXAMPLE 2 Twisted molybdenum filament (gradient apparatus) A ten inch length of five mil molybdenum wire obtained from Fansteel Metallurgical Corporation was subjected to a torsional stress of ten turns per inch and then placed in the envelope a which was evacuated to a pressure of the order of 10* millimeters of mercury.

The source a was adjusted to heat the twisted filament to a temperature of 2000 K. and the apparatus 28 was adjusted to provide a lowering of the level of the mercury 26 of slightly more than one centimeter per hour, the heating being maintained for approximately twelve hours. This filament exhibited a pronounced axially extending spiral of high electron emission in the projection pattern having a pitch of ten turns per inch. Further, clearly identified dark and bright bands of emission along the 110, 111, and 116 crystal planes of the projection pattern indicated a single crystal extending the full length of the filament.

EXAMPLE 3 Twisted tungsten filament (Robinson apparatus) A ten inch length of unpolished five mil tungsten obtained from General Electric Company, Nela Park, which is identified as GE218, was subjected to a torsional stress of five turns per inch and placed into the envelope 10. The pressure in the envelope was reduced to the order of 10" millimeters of mercury by the pump 24, and the source 20 was adjusted to heat the twisted filament to a temperature of 2500 K. This heating was maintained for around twelve hours after which a study of the bright and dark bands of the projection pattern along the 110, 111, and 116 crystal planes indicated a single crystal having a length of four inches. A pronounced axially extending spiral of high emission having a pitch of five turns per inch was also observed in the projection pattern when the filament was heated.

EXAMPLE 4 Twisted tungsten filament (gradient method) A ten inch length of five mil tungsten wire obtained from General Electric Company, Nela Park, and identified as GE218 was subjected to a torsional stress of ten turns per inch and then placed in the envelope 10a. The pressure within the envelope 10a was reduced to a magnitude of the order of 10* millimeters of mercury and the twisted filament was heated to a temperature of 2800 K. This heating was'continued for twelve hours with a mercury removal rate that lowered the level of the mercury 26 slightly more than one centimeter per hour. The twisted filament 8 exhibited a spiral pattern of high electron emission having a pitch of ten turns per inch. This filament also provided a projection pattern showing clearly identified bright and dark bands of emission along the 110, 111, and 116 crystal planes indicating a single'crystal of ten centimeters in length.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electron emitting element comprising a filament of refractory metaltwisted about its longitudinal axis and having an axially extending spiral band of high electron emission.

2. An electron emitting element comprising a filament of refractory metal twisted about its longitudinal axis and consisting of a single crystal, said filament having an axially extending spiral band of high electron emission.

3. An electron emitting element comprising a filament Cir of refractory metal twisted about its longitudinal axis in the range of from one to twenty turns per inch and having a spiral band of high electron emission.

4. The element set forth in claim 3 in which the refractory metal is tungsten.

5. The element set forth in claim 3 in which the refractory metal is molybdenum.

6. An electron emitting element comprising a filament of refractory metal having an axially extending spiral band of high electron emission.

7. An electron emitting element comprising a single crystal of refractory metal having spaced screw planes passing therethrough providing a spiral band of high electron emission.

8. Theelement set forth in claim 7 in which the screw planes consist of an electropositive impurity material.

9. A method of recrystallizin'g refractory metal comprising the steps of twisting an elonged mass of refractory metal, and heating said mass of refractory metal at a temperature in the range from 1300 K. to 2800 K. for a period of time long enough to develop single crystal growth in said mass.

10. A method of forming electron emitting elements comprising twisting a length of refractory metal about its longitudinal axis, and heating said twisted length of refractory metal in vacuum at a temperature in a range from 1300 K. to 2800 K. for a period of time long enough to develop single crystal growth.

11. The method set forth in claim 10 in which the refractory metal is twisted between one and twenty turns per inch.

12. A method of forming refractory metal filaments comprising the steps of twisting a filament of molybdenum about its longitudinal axis to provide between one and twenty turns per inch, and heating said twisted filament in vacuum to' a temperature in the range between 1300 K. and 2000 K. for a period of time long enough to develop single crystal growth.

13. A method of forming refractory metal filaments comprising the steps of twisting a filament of tungsten about its longitudinal axis to provide between one and twenty turns per inch, and heating said twisted filament in vacuum to a temperature in the range of from 2200" K. to 2900 K. for a period of time long enough to develop single crystal growth.

14. A method of forming refractory filaments comprising the steps of twisting a filament of molybdenum about its longitudinal axis to proivde four turns per inch, and heating said twisted filament in vacuum at a temperature of 1650 K. for a period of time long enough to develop single crystal growth.

15. The method set forth in claim 10 in which the step of heating said twisted length of refractory metal comprises moving a temperature gradient along said twisted length.

16. The method set forth in claim 14 in which the step of heating the twisted filament comprises moving a temperature gradient of substantially 1650 K. along the length of said twisted filament.

17. The method set forth in claim 16 in which the temperature gradient is moved along said twisted filament at a rate of substantially one centimeter per hour.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Nov. 10, 1927 

9. A METHOD OF RECRYSTALLIZING REFRACTORY METAL COMPRISING THE STEPS OF TWISTING AN ELONGED MASS OF REFRACTORY METAL, AND HEATING SAID MASS OF REFRACTORY MATAL AT A TEMPERATURE IN THE RANGE FROM 1300* K. TO 2800* K. FOR A PERIOD OF TIME LONG ENOUGH TO DEVELOP SINGLE CRYSTAL GROWTH IN SAID MASS. 